The present invention concerns a process for the preparation of aminophenols by the hydroxylation, in liquid superacid medium utilizing hydrogen peroxide, of certain aromatic amines.
The invention allows the introduction of a phenol function on the aromatic ring of the amine in the ortho, meta, and para positions in relation to the amine group, without oxidation of the nitrogen of the amine functional group. The proportion of the meta-aminophenol formed is predominant.
French Pat. No. 82 10644 describes a process for the hydroxylation of anilides; i.e., amides derived from aniline, with hydrogen peroxide in a superacid medium. It is particularly surprising that an analogous process can be applied to unprotected anilines, since prior art very clearly teaches that peroxide compounds (hydrogen peroxide, organic peracids, hydroperoxides, diacyl peroxides) in general attack the nitrogen of the amine function with the formation of hydroxylamine, amine oxide, nitroso derivatives, azo derivatives, azoxy derivatives or polymers. Sometimes an attack on the alkyl groups borne by the nitrogen of secondary and tertiary aromatic amines is observed involving dealkylation reactions or the formation of imines.
These different modes of action of peroxide compounds on aromatic amines are described in the following references:
B. C. CHALLIS and A. R. BUTLER, in "The Chemistry of the Amino Group" (PATAI), p. 320-338, Interscience Publishers, New York, 1968; PA1 M. HEDAYATULLAH, Bull. Soc. Chim. France, 1972, p. 2957-2974; and PA1 J. P. SCHIRMANN and S. Y. DELAVARENNE, "Hydrogen Peroxide in Organic Chemistry", chapter VII, p. 147-156, EDI Paris, 1979.
In the special case of N,N-dialkylanilines, oxidation by peroxydisulfate ions, known under the name of BOYLAND-SIMS reaction, leads to the introduction on the aromatic ring of a sulfate group, later hydrolyzable into a phenol group. This introduction essentially takes place in the ortho position with respect to the amine group, with the para isomer forming in significant proportions only when the two ortho positions are blocked. No meta isomer is formed. This BOYLAND-SIMS reaction has been studied by E. J. BEHRMAN and D. M. BEHRMAN, J. Org. Chem., 43, No. 23, p. 4551-4552 (1978).